Surprises in the Crab Nebula Print E-mail

ImageIs the famous Crab Nebula a reliable standard candle? An interview with Marco Tavani (INAF, Rome), Principal Investigator of the Italian AGILE X-ray and Gamma ray astronomical satellite.

Did you get data from other observatories as well?

During the flare in September last year we urged colleagues around the world to look very fast at this phenomenon. The Hubble Space Telescope reacted within about ten days with a nice picture. The Chandra X-ray satellite reacted within one week. The Swift satellite, which works very well for catching Gamma Ray Bursts, reacted within a few hours. But Swift did not detect any variation. For detailed imaging of the inner nebula, you need a gigantic observatory like Hubble or Chandra. So Swift could detect the overall X-ray flux of the pulsar and determined that the pulsations from the pulsar were regular. The pulsar signal obviously did not change.

Are there any models about how particles can be accelerated so fast?

That’s the big question. The pulsar is a fantastic accelerator pushing particles to energies a factor of 1000 or so higher than at the Tevatron or the Large Hadron Collider. The gamma ray flares are very fast. From the time scale of the variation you can demonstrate that the acceleration must occur in a small region. It can not be very distant from the pulsar. There are different sites in the nebula now identified by Hubble and Chandra, which are the main suspects to identify the real region from where this gamma ray flaring is originating.

What do you know about these suspicious regions?

There are two regions in the Crab, where particles are actually accelerated: One is close to the pulsar, that is a very complicated electromagnetic machine where the pulsed emission originates. Energetic particles stream out in a very asymmetric way. They are concentrated in a torus-like structure which is believed to be the equatorial plane of the pulsar. In addition, there are two “jets”. The word jet is not really correct in this case, but people call them jets, because definitely they are channels of emission that you can detect both in the optical and in the X-ray regime. They come from the polar regions of the pulsar. There is this dichotomy in the pulsar particle wind: we have a a torus-like shape and a jet-like region.
Are there any models for the acceleration mechanism itself?

Not yet. The observed very fast acceleration seems to contradict what people believed so far to be the main shock mechanism of acceleration in the nebula, which is called diffuse shock acceleration.  >>

When particles in the shock go upstream and downstream many times they gain net energy. The trouble with that is that the particles can get energy, but also they can cool very rapidly because of the magnetic field in the region. In the end there must be some sort of balance between the maximum energy you can get from this kind of going back and forth in the shock and the cooling processes by synchrotron radiation. In the end you can demonstrate that the maximum energy you can get in this sort of naive picture is about 50 MeV. Clearly, with energies now observed as high as a few GeV we shattered this naive picture completely.

Are you still observing the Crab Nebula?

We are currently working with our collaborators across the globe to monitor the Crab Nebula intensively for additional flares. Next time we want to identify the regions of the nebula where these things happen. We definitely need the “giants” like Hubble and Chandra. We hope to convince them to look at this phenomenon faster than before. This is going to be very tough because these satellites usually are not operated on a short time scale.

Is this more an issue of astrophysics or particle physics?

It is both. AGILE and the Fermi satellite use detectors that are more or less the same as at CERN or Tevatron, but of course with a completely different twist. Cosmic phenomena are unpredictable. It is not like being in your lab where you control everything of your equipment and where you can run experiments. In astrophysics you have to be patient and wait some time. Then you can really study these fantastic acceleration processes. On top of that there is some connection with plasma physics because, after all, these cosmic media are just plasmas. The understanding of the instabilities and the acceleration of these plasmas can even lead to a better understanding of fusion physics. I hope the communities of particle, astro and plasma physics find a common ground and go back to the blackboard to study these phenomena together.

Crab NebulaThe Crab Nebula through the eyes of the VLT

The Crab Nebula is among the most remarkable sources known to astrophysicists. It is the impressive remnant of a supernova that exploded in 1054 AD, and is a very bright X-ray and gamma-ray source. At its centre lies a pulsar (a neutron star rotating 30 times a second) producing a very energetic electromagnetic wind of waves and particles. The Crab system (the pulsar and nebula) provides an ideal laboratory for studying some of the most mysterious physical processes in the Universe. The strength and other unique characteristics of the Crab (including its apparent stability) have led astronomers to use it as a standard reference for X-ray and gamma-ray measurements.
Submitted by Dirk Lorenzen (Germany)
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